Synchronous linear motor

ABSTRACT

To improve the movement precision of a synchronous linear motor, it is proposed that the front and rear end faces (11, 12) of the primary section (10) be ungrooved and not wound and bevelled in sections in such a way that the angle of inclination (β) of the bevelled surfaces (11a, 12a) on the front and rear end faces (11, 12) of the primary section (10) in relation to the longitudinal axis (13) of the motor is selected according to the relation: β=arctan (b/τ p ) where β is the angle of inclination of the bevelled surfaces (11a, 12a) at the front and rear end faces (11, 12) of the primary section (10), b is the electrically active width of the primary section (10) and τ p  is the pitch of the poles (21, 22) of the secondary section (20).

BACKGROUND OF THE INVENTION

The invention relates to a synchronous linear motor of a type includinga primary section having rotor grooves for receiving a single-phase ormulti-phase rotor winding, and a secondary section including a sequenceof permanent magnets, each of the magnets having two pole pairs actingas north and south poles, with the length of the secondary section inthe moving direction exceeding the length of the primary section. Asynchronous linear motor of this type is known from U.S. Pat. No.4,908,533.

High quality synchronous motors for applications as actuating motorsshould generate forces rather uniformly and without malfunctioning. Inrotating synchronous motors, periodic variations in force ("forcewaviness") occur which are mainly caused by grooves disposed on thestator. In order to compensate for the force waviness as well as for allother effects on the drive shaft torque caused by the grooves, the rotorand stator poles are usually beveled across the width of a groove.

There is also known from U.S. Pat. No. 4,908,533 for synchronous linearmotors, that the force waviness can be reduced by beveling the polesacross the width of a groove of the wound primary section. Since in topplan view, the edges of the faces of the primary section are parallel tothe grooves in the primary section, both the front and rear faces of thepoles are beveled in the conventional beveling operation of the grooves.The force waviness can also be reduced by a method known from EP 0 334645 A1, wherein the core of the primary section of a synchronous linearmotor is formed as a ferromagnetic plate and coils are placed in the airgap of the linear motor in such a way that the face regions of the plateproject beyond the air gap coils and form a step along the longitudinalmedian line of the linear motor.

In contrast to rotating synchronous motors which continue indefinitelywhen viewed along the circumference, the synchronous linear motor isspecial in that it has a beginning and an end. At the transition pointsat the beginning and the end of a synchronous linear motor, there aregenerated periodic motor end forces in the moving direction whichadversely effect the operation of the linear motor. The motor end forcesare generated because the linear motor covers the magnetic polesdifferently, depending on the motor position. As a result, there existpreferred positions where the stored magnetic energy of the linear motoris particularly large. Additional forces are then required in order tomove the linear motor away from these preferred positions. The forcewith which the linear motor pulls itself into the preferred positions ofthe magnetic poles, is called "pole force." The pole force can reach upto approximately 25% of the rated motor force. There exists a preferredposition above each magnetic pole. Consequently, the pole force has thesame periodicity as the magnetic poles and interferes with the motorforce; this phenomenon is referred to as "pole waviness." Since the poleforce does not depend on the motor current, it represents a passiveforce which is also present in the absence of current. The pole forcedoes not perform any work, because it operates alternating in the movingdirection and opposite to the moving direction of the linear motor.During operation, the pole force is added to the force generated by themotor current. The pole force has nothing in common with the grooveforce which is the force describing the interaction between the edges ofthe magnetic poles and the stator grooves.

In the conventional synchronous linear motor, the aforedescribed "polewaviness" causes an imprecise movement which is particularly undesirableif such motors are used as precision actuators.

SUMMARY OF THE INVENTION

Therefore, it is the object of the invention to improve the motionaccuracy of a synchronous linear motor of the abovedescribed type.

The object of the invention is attained by providing the primary sectionwith front and rear end faces which are free of grooves and free of awinding and exhibit areas extending perpendicular to an air gap zonebetween the primary section and the secondary section and being beveledsuch as to define an angle of inclination which is selected with respectto the longitudinal axis in accordance with the following relationship:

    β=arctan (b/τ.sub.p),

wherein

β is the angle of inclination of the beveled areas at the front and rearend faces of the primary section,

b is the electrically active width of the primary section, and

τ_(p) is the pitch of the poles of the secondary section.

Preferred embodiments and improvements of the synchronous linear motoraccording to the invention appear in the dependent claims.

In the synchronous linear motor of the invention, both motor ends arebeveled by the width of a magnetic pole. The beveled motor end region isneither grooved nor does it have a winding. In contrast to theconventional measures for eliminating groove forces described above, thesynchronous linear motor of the invention is not changed in the regionsof the linear motor winding . In the present invention, both motor endsare beveled by the width of a magnetic pole; consequently, each poleforce components on the front face of the linear motor is matched by anexactly identical pole force components at the rear face of the linearmotor. In order to compensate the pole forces completely, the motor endhas to be beveled across the entire width of the magnetic pole. The poleforce compensation will be insufficient if the faces are beveled to alesser or greater extent. This is the reason why the pole waviness isnot eliminated when in order to prevent groove disturbances, the groovesare beveled in the aforedescribed manner, and the motor faces aresimultaneously beveled parallel by about one groove interval. Ingeneral, three or six grooves, respectively, cover the magnetic pole ofa linear motor which is equivalent to beveling the motor faces onlyacross one third or one sixth, respectively, of a magnetic pole, whenbeveled conventionally over one groove.

If the magnetic poles of the secondary section extending over the widthof one groove are to be beveled in order to compensate thegroove-induced force waviness, then according to the invention, thebevel of the end faces of the primary section has to be increased ordecreased by the bevel of the magnetic poles, depending on the directionof the bevel on the end faces, i.e. if the magnetic poles are beveled inthe same direction as the faces of the primary section or in thedirection opposite to the direction of the front faces of the primarysection.

The end faces of the motor which, according to the invention, arebeveled over the width of a magnetic pole, have no winding and thus donot require grooves. The best result is achieved when the bevel of thefaces of the primary section towards the secondary section forms asmooth surface with the same air gap as the linear motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described hereinafter with reference toembodiments shown in the drawings. It is shown in:

FIG. 1 a schematic top plan view of a first embodiment of a synchronouslinear motor according to the invention;

FIG. 2 a top plan view of a second embodiment of a synchronous linearmotor according to the invention, wherein--in contrast to FIG. 1--thepole gaps of the secondary section are beveled;

FIG. 3 a top plan view of a third embodiment of a synchronous linearmotor according to the invention, wherein--in contrast to FIG. 1--therotor grooves are beveled; and

FIG. 4a perspective view of an attachment formed according to theinvention for the front and rear face of a primary section.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In FIG. 1, there is shown a top plan view of a synchronous linear motor1 according to the invention, consisting in the conventional manner of aprimary section or rotor 10 and a secondary section 20. The movingdirection of the linear motor 1 is indicated by an arrow 30. The lengthofthe primary section 10 in the moving direction 30 is less than thelength of the secondary section 20. The primary section 10 includes astratified sheet metal body 16 with parallel extending rotor grooves 14,of which rotor grooves 14 only two are indicated by II dotted lines atthe left edge of the sheet metal packet 16. In the example of FIG. 1,the longitudinal axes of the rotor grooves 14 extend perpendicular tothe longitudinal axis 13 of the primary section 10 and the coincidinglongitudinal axis 23 of the secondary section 20. Alternatively, asshown in FIG. 3, the rotor grooves 14 may also be formed at an angle ofinclination which is different from 90° with respect to the longitudinalaxis 13 of the primary section 10. In the rotor grooves 14, there isdisposed a two-phase or three-phase rotor winding, which is electricallyexcited in a manner not shown here by a two-phase or three-phase ACvoltage.

The, for example, stationary secondary section consists of a pluralityof permanent magnets arranged successively in the moving direction, witheachof the permanent magnets having a north pole 21 and a south pole 22.Between the poles 21, 22 of each pole pair 21/22 having a width W₁ thereis disposed a small pole gap 24 having a gap width s. The longitudinalaxes of the pole gaps 24 in the embodiment of FIG. 1 extendperpendicular to the longitudinal axis 13 of the primary section 10 andare thus oriented in the same direction as the longitudinal axes of therotor grooves 14.

When the rotor windings 15 are excited, an electro-motoric force isinducedwhich moves, for example, the primary section 10 securedunderneath a carriage relative to the stationary secondary section 20 inthe direction of the arrow 30. The motion of the primary section 10 issynchronous with the frequency of the two-phase or three-phase ACvoltage exciting the primary section 10, which gives this type of linearmotor the name synchronous linear motor.

According to the invention, the end faces 11, 12 of the sheet metalpacket 16 of the primary section 10 are formed ungrooved and are beveledby the width W of a magnetic pole 21, 22, thereby forming an angle ofinclination β with respect to the longitudinal axis 13. This bevel ofthe front faces 11, 12 is formed either over the entire end faces 11a,12a, as shownin FIG. 1, or only over a section thereof, as shown in theform of an attachment element 100 in FIG. 4. The ungrooved attachmentelement 100 is secured to each axial end of the sheet metal body 16,thereby allowing manufacture of the sheet metal body 16 in aconventional fashion with grooves 14 and windings 15. Each attachmentelement 100 has the sheet metal arranged perpendicular to the air gapzone between the primary section 10 and the secondary section 20,wherein the orientation of the stratified sheet metal is preferably thesame as the orientation of the sheet metal body 16. In the embodiment ofFIG. 4, the end face 110 has a beveled face section 111 and anunbeveled, straight face section 112, witha horizontal step with atriangular cross-section disposed between the facesections 111 and 112.The height of the beveled face section 111 is, for example, more thanfive times the height of the air gap between the primary section 10 andthe secondary section 20. Such partial bevel of theend faces 11a, 12a(which can not only be attained with attachment elements100, but alsowith a single-piece design of the sheet metal body 16) increases themechanical strength of the sheet metal attachment elements 100. Theattachment elements 100 can be formed in such a way that they canbescrewed directly to the ends of the rectangular sheet metal packet 16.In addition, the attachment elements 100 can be secured to the samemachine component (not shown) as the sheet metal packet 16. In thiscase, the attachment holes (not shown) for the sheet metal packet 16 areextended up to the attachment elements 100.

If the faces of the sheet metal packet 16 are made of a single piece,then the individual core sheets of the grooved sheet metal packet 16 canhave an ungrooved projection of different length. When the individualsheets are assembled into the sheet metal packet 16, then theprojections of different length form the beveled faces of the primarysection 10. If it is too costly to fabricate individual sheet metalsections with different length in order to obtain a beveled face on theprimary section 10, then the bevel can also be formed by milling thefinished sheet metal body 16. Hereby, it is sufficient if the bevelextends only over a portion of the height of the sheet metal packet 16,as is depicted in FIG. 4 for the attachment elements 100.

In the further embodiment of the synchronous linear motor of theinvention,shown in FIG. 2, the magnetic poles 21, 22 of the secondarysection 20 are beveled at an angle δ perpendicular to the movingdirection 30 for the purpose of compensating the groove-induced forcewaviness. The magnetic poles 21, 22 can be beveled, as shown in FIG. 2,in the directionof the bevel of the end faces 11, 12 of the primarysection 10. However, itis also feasible to bevel the magnetic poles 21,22 (in a manner not shown here) in the opposite direction of the bevelof the end faces 11, 12 of the primary section 10. If the magnetic poles21, 22 are beveled by an angle δ in the same direction as the bevel ofthe end faces 11, 12, then the angle of inclination β of the end faces11, 12 is increased by the angle of inclination β of the magnetic poles;if the magnetic poles 21, 22 are beveled by an angle 13 in the oppositedirection of the bevel of the end faces 11, 12, then the angle ofinclination b of the end faces 11, 12 is decreased by the angle ofinclination δ. The following relationship applies to the angle ofinclination β of the end faces 11, 12 of the primary section 10 in thecase where the magnetic poles 21, 22 of the secondary section 20 arebeveled by an angle δ:

    β=arctan (b/τ.sub.p)±δ,

wherein

β is the angle of inclination of the beveled surfaces 11a, 12a at theend and rear front regions 11, 12 of the primary section 10,

b is the electrically active width of the primary section 10,

τ_(p) is the pitch of the poles 21, 22 of the secondary section 20, and

δ is the bevel angle of the magnetic poles 21, 22 of the secondarysection (20) which is inserted in the above equation with a positivesign if the magnetic poles 21, 22 are beveled in the same direction asthe end faces 11, 12 of the primary section 10, and with a negative signif the magnetic poles 21, 22 are beveled in the opposite direction ofthe end faces 11, 12 of the primary section 10.

It is understood that the longitudinal axes of the rotor grooves 14according to FIG. 2 and the pole gaps 24 according to FIG. 3 can bebeveled concurrently. It is also possible, either instead of or inaddition to beveling the pole gaps 24, not to maintain a constant gapwidth s (as depicted in the FIGS. 1 to 3), but to instead change the gapwidth s continuously, resulting in conical pole gaps 24.

What is claimed is:
 1. A synchronous linear motor comprising:a primarysection having a width and a length, said primary section being formedwith rotor grooves for receiving a rotor winding; and a secondarysection defining with the primary section a longitudinal axis and havinga length which in a moving direction exceeds the length of the primarysection, said secondary section including a sequence of permanentmagnets, each of the magnets having two pole pairs acting as north andsouth poles and spaced from one another at formation of a pole gap, saidprimary section having front and rear end faces which are free ofgrooves and free of a winding and exhibit areas extending perpendicularto an air gap zone between the primary section and the secondary partand being beveled such as to define an angle of inclination which isselected with respect to the longitudinal axis in accordance with thefollowing relationship:

    β=arctan (b/τ.sub.p),

wherein β is the angle of inclination of the beveled areas at the frontand rear end faces of the primary section, b is the electrically activewidth of the primary section, and τ_(p) is the pitch of the poles of thesecondary section.
 2. The synchronous linear motor of claim 1 whereinthe rotor grooves of the primary section have, with respect to thelongitudinal axis, an angle of inclination which is different from 90°.3. The synchronous linear motor of claim 1 wherein the rotor grooves ofthe primary section are oriented perpendicular with respect to thelongitudinal axis.
 4. The synchronous linear motor of claim 1 whereinthe pole gap between each one of the pole pairs of the secondary sectionhas, with respect to the longitudinal axis, an angle of inclinationwhich is different from 90°.
 5. The synchronous linear motor of claim 1wherein the angle of inclination of the end faces of the primary sectionis selected, at inclination of the magnetic poles of the secondarysection about an angle, in accordance with the following relationship:

    β=arctan (b/τ.sub.p)±δ,

wherein β is the angle of inclination of the beveled areas of the frontand rear end faces of the primary section, b the electrically activewidth of the primary section, τ_(p) the pitch of the poles of thesecondary section, and δ the angle of inclination of the magnetic polesof the secondary section which in the above equation has a positive signwhen the magnetic poles are beveled in the same direction as the endfaces of the primary section, and has a negative sign when the magneticpoles are beveled in the opposite direction of the end faces of theprimary section.
 6. The synchronous linear motor of claim 1 wherein thepole gap between each of the pole pairs of the secondary section has avariable gap width.
 7. The synchronous linear motor of claim 1 whereinthe pole gap between each of the pole pairs of the secondary section isoriented perpendicular to the longitudinal axis.
 8. The synchronouslinear motor of claim 1, and further comprising attachment elements madeof magnetically conducting material for securement to the front and rearend faces of the primary section, each one of the attachment elementshaving at least one beveled free end face.
 9. The synchronous linearmotor of claim 8 wherein the attachment elements are formed offerromagnetic metallic sheets which are oriented perpendicular to themoving direction of the linear motor.
 10. The synchronous linear motorof claim 1 wherein the rotor winding is a single-phase rotor winding.11. The synchronous linear motor of claim 1 wherein the rotor winding isa multi-phase rotor winding.